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CoaXPress

CoaXPress (CXP) is the second beyond CameraLink standard that was
established a few years ago. In the simplest version, power, control and
data for the remote device are send via a single coaxial cable.

Coaxial cables were in common use during the initial years of imaging
for analogue cameras, providing a cost-efficient and robust solution
with marginal signal loss over long cable lengths. In the course of the
digital (r)evolution, these traditional cables are now also making inroads
in both, CoaXPress digital transmission standard and also the HD-SDI
broadcast standard. For this, a parallel digital signal which is generated
by the camera is serialised and then send to the frame grabber through
a single 75 ohm coaxial line in data packages at a correspondingly
higher frequency and with electronic conditioning. Due to the fact that
CoaXPress, unlike IEEE 1394 or Gigabit Ethernet, is not using standard
mass market technologies, the CoaXPress specification comprises the
electrical specification of the signals on the cable, in addition to the
protocol definition. CoaXPress establishes a point-to-point link between
the camera and the frame grabber, so that the complete bandwidth is
always available to the image data of the camera.

As described, in the simplest case, CoaXPress uses a single coaxial line
for forward and return channels, so that it is fairly easy to route the
connection of the camera through a slip ring to provide a movable
camera installation with little effort. Unlike in the case of GigE Vision or
IEEE 1394, a frame grabber is needed which receives the data in the PC
and sends control data to the camera. The drawback of the frame
grabber is that an additional card must be installed in the PC; its
advantage is that extra preprocessing can be integrated on it and higher
bandwidth which was the original intention of CXP.

Coaxial cables, legacy from many older analogue installations, make
conversion to digital technology with higher bandwidth a simple and
low-cost effort. However, special care should be taken as not all old
cables can be used for CoaXPress, please get in touch for detailed
advice.

CoaXPress-Setup

To ensure data integrity, CoaXPress uses a CRC checksum for the image
data. Unlike, e.g. with CameraLink, bit errors can be detected during the
transmission with this arrangement. However, to reduce the level of
complexity at the receiving end and not having to install additional
memory capacity in the camera, no resend mechanism, (by which
corrupt packages can be resent to the frame grabber), is provided.

The camera is triggered either via an input on the camera or via the
20 Mb/s uplink from the frame grabber used for camera control. As a
result of this, trigger signals have a fixed latency of 3.4 µs with a jitter of
only 4 ns.

The connector in the one-cable solution is a commercial coaxial BNC or
DIN 1.0/2.3 plug. More and more frame grabber manufactures are now
using the DIN connector.

Different connector types

A single cable can transmit up to 6.25 Gb/s from the device to the frame
grabber and 20 Mb/s of control data from the frame grabber to the
device which - as far as image data is concerned - is equal to
approximately 5 or 6 times the GigE bandwidth. However, some
CoaXPress cameras and frame grabbers in the market can run at half
speed 3.125 Gb/s.

The voltage supply to the device is provided through a CoaXPress link
24 V at a maximum of 13 watt, which is sufficient for many cameras,
however, for more demanding power needs, the camera can be supplied
with voltage in the conventional manner. The maximum cable length is
25 m at the full bandwidth of 6.25 Gb/s, or 65 m at half the bandwidth
of 3.125 Gb/s (at time of writing).

CoaXPress Setup

If a single link is not sufficient, several cables can be combined to use
several links in parallel to reach higher bandwidth. At the time of
writing cameras and frame grabbers are available with 4 parallel links.

The software support of CoaXPress also strikes out in a new direction
being the first standard to require the provision of a GenICam GenTL for
the frame grabber. As with the other modern machine vision camera
standards, CoaXPress must provide a GenICam GenApi XML file with
descriptions of the features provided by the camera. This makes
CoaXPress the first standard in industrial imaging that provides a
uniform software interface for enumerating and controlling cameras.

CoaXPress was presented by a consortium of firms at VISION 2010 for
the first time. Today this standard is hosted by JIIA, the Japan Industrial
Imaging Association, a member of the G3 consortium of AIA (Automated
Imaging Association), EMVA (European Machine Vision Association)
and JIIA. The specification of CoaXPress is freely available in the web. For
further information, see http://www.jiia.org or www.coaxpress.com.